Blazing fiery descent of space-borne objects into the Earth atmosphere is a well known phenomenon. To avoid damage or destruction during the fiery reentry phase, reentry vehicles are usually equipped with heat-shields known as the thermal protection system (TPS). With a TPS shield, it is the outer layer of TPS, rather than the structure of the reentry vehicle, that is subject to the intense heat and pressures during reentry. However, the TPS itself is not indestructible. For instance, the outer layer of TPS ablates away under the extreme heat and pressures of reentry phase. As the TPS chars and partly burns away, a layer of plasma is created around the reentry vehicle from the ablation process.
Plasma is a state of matter in which molecules or atoms are broken down to free electrons and positively charged ions—atoms with electrons stripped away—under extreme heat. The TPS ablation releases certain easily ionizable trace elements in the TPS—mainly the low ionization potential alkali metals such as Na, K, and Ca—into a thin layer of flow around the reentry vehicle. A significant portion of these atoms ionize when they are subject to the high temperatures that exist in the thin boundary layer of flow around the reentry vehicle. Equal numbers of singly ionized positive ions (Na+, Ca+, and K+) and free electrons (e−) are generated, and an overall charge balance is maintained. Although these trace elements are usually present only at very low levels in the TPS material, on the order of tens of parts per million, they make a large contribution to the free electron concentration in the vehicle boundary layer.
Another source of plasma is from ionization of the air molecules, primarily in the region of the nose tip shock wave. In this region just at the front of the vehicle, the bow shock wave is at right angle to the flow and produces much higher temperatures than the flow along the sides of the vehicle. Immediately behind this right angle shock wave, the air is highly dissociated and partially ionized to a mixture of N, O, NO, N2, O2, NO+, and e− molecules and ions. The most easily ionizable air derived species is NO, making creation of NO+ the principal source of “clean air” ionization free electrons. While not present in ambient air in significant amounts, NO is formed rapidly when the air is dissociated by the high temperatures at the nose tip. The relatively intense plasma formed at the nose tip flows back along the vehicle sides and eventually around the back into the wake, thinning as it goes and mixing with the boundary layer flow with its alkali metal produced ions. Hence, there are two major plasma contributors, one from the nose tip bow shock wave and the other from the ablating TPS layer with its alkali trace elements.
One result of plasma formation around a reentry vehicle is communication disruption during reentry. The free electrons racing around in the plasma interfere with the electromagnetic waves used for radio communication (the RF signals), resulting in communication disruption. The communication blackouts of space capsules or space shuttles that occur during their reentry into the Earth atmosphere are caused by this plasma effect.
Communication disruption during reentry affects reentry vehicles in many ways. For example, the plasma effect adversely affects the performance of reentry vehicles with missile launch capability. One method of maintaining the performance of the reentry vehicles is to guide the reentry vehicles utilizing a GPS (global positioning system). GPS is typically used to control various accuracy parameters required for low-yield nuclear or conventional payloads carried within reentry vehicles, such as, angle of attack, flight path angle, and angle of obliquity requirements, in order to achieve maximum mission effectiveness. However, the plasma effect disrupts the vehicles' communication with the GPS satellites, causing so-called GPS blackouts during reentry. Because the reentry vehicles cannot be guided by GPS during the critical reentry phase, performance is negatively affected by the plasma induced communication disruption.
Prior methods for controlling reentry vehicles during the GPS blackout using flaps, fins, moveable nose tips and small control rockets have been developed. Other exotic, difficult to design and manufacture high-lift shapes have also been proposed at the concept level.
The technology for reducing or suppressing plasma effects, thus, would provide numerous benefits including, for example, making space flights safer and making missiles more accurate. Hence, it would be desirable to provide methods and systems that are capable of reducing plasma induced communication disruption during reentry.